HF (high frequency) radio, used today mostly in military applications, has various benefits. These include its low cost, its range, and its robustness. Its main disadvantage is that signal quality and propagation continually vary with time and location. Long distance communication using HF radios normally relies on ionospheric reflection of the signal, thus allowing beyond-line-of-sight communication. Unfortunately, ionospheric propagation is not consistent. Quality varies with operating frequency, location, signal direction, seasons of the year, and time of day. Distortion and interference can limit the effective rate of data transfer. Nevertheless, HF radio has appeal to users with long haul communications requirements but without access to satellite facilities.
Technological advances in HF modem design, Automatic Link Establishment (ALE) and frequency management have allowed automated HF systems to be built.
When multiple channels are available, ALE is a communications technique based on assigning an ALE adaptive controller the task of automatically controlling a high frequency (HF) receiver and transmitter in order to determine and establish the highest quality communications link with one or more HF radio stations. ALE automates the process of searching channels to find one that offers the optimum signal quality. In the majority of existing ALE HF systems, the participants in a communication net or link are assigned a common set of frequencies. The role of the automated system is to establish a useable link between participants on one of these frequencies.
In asynchronous ALE systems, radio receivers at different ends of the HF link scan round the common set of frequencies, stopping on each for a set period of time to determine if a link request is being made on that frequency. The different radios have no common knowledge of time, and hence scan round the frequency group asynchronously. When the ALE system wants to set up a link to another user, it selects a frequency from the predefined set and transmits a known calling sequence on that frequency to request a response from the intended destination. Due to the asynchronous nature of these systems, the transmission time for the call request needs to be long enough to ensure that the called station has sufficient time to scan round all the frequencies in the scan group. The calling station therefore transmits the calling sequence for a sufficiently long period and then, if a response is heard on the called frequency, it is assumed that the link is established. If no response is heard, the calling station chooses another frequency from the frequency group and continues with a new calling sequence until a frequency is found that can be used for link set up.
ALE systems may also use synchronous communications, which are conventionally implemented with internally generated synchronization signals. In one implementation, one station is designated as ‘master’ and the others synchronize to it. In operation, stations in a radio net scan round the common set of frequencies and listen on each frequency for a predetermined time for a call request. In this case the radio stations in the net are all scanning the same set of frequencies at the same time and, since the calling station knows what frequency the called station is listening on, link set up is faster and on-air time shorter than is the case with an asynchronous system.
Although ALE is a powerful communications technique, it suffers from high signal overhead, which reduces information throughput. Signal acquisition time, including modem training intervals, contributes to an inordinate amount of time spent setting up a communication channel for transmission of desired information. Most ALE systems use non-coherent rather than coherent modulation techniques, because of synchronization difficulties and the fact that coherent synchronization requires both frequency and phase synchronization.